An epithelial cell polypeptide growth factor has been partially purified from conditioned media from a human embryonic lung fibroblast cell line, M426. Poly A+ RNA from M426 fibroblasts has been isolated and used to construct a cDNA expression library in the Okayama-Berg plasmid vector, pcDV1, which promotes expression of the cloned cDNA in mammalian cells. This library will be used to screen for expression of the cDNA coding for the mitogen using COS cells as hosts. The latter are capable of greatly amplifying transfected DNAs and therefore the amount of gene product synthesized.